The New International Encyclopædia/Steam Navigation
STEAM NAVIGATION. The Spaniards assert that as early as 1543 Blasco de Garay made an attempt to propel a vessel by steam in the harbor of Barcelona. In the absence of direct proof of the fact this may well be doubted. At the time mentioned the most advanced scientists in Europe had not yet begun seriously to consider steam as a source of power. The assertion is also made that Denis Papin (q.v.) in 1707 propelled a boat by steam on the River Fulda. Papin invented the safety valve and a single-acting steam cylinder pump, and made various improvements in steam pumps, but it does not appear that he ever built what might be called a steam engine. The boat which has been mentioned and which is frequently referred to had some sort of paddle wheels, but they were operated by the crew and not driven by steam power. In 1729 Dr. John Allen took out a patent in England for a method of propelling a boat by means of forcing water out of the stern with steam or other pressure. In 1736 the rather vague ideas of Allen were improved upon by Jonathan Hulls, a clockmaker of Campden, England, and he was granted a patent for mechanism to propel a boat by steam power. Like Allen, he apparently made no serious attempts to put his ideas into practice. In 1752 the French Academy of Sciences awarded a prize to the distinguished physicist Daniel Bernoulli for an essay on the manner of propelling boats without wind. In addition to other suggestions he proposed the use of the screw propeller.
(From an old drawing.)
Up to this time successful steam navigation was impossible because a practical steam engine did not exist. This deficiency was supplied by Watt, who took out his first patent in 1769, but the engines contemplated were really single-acting pumps. In 1782, however, Watt brought out the double-acting engine, and developed the principle of expansive working by cutting off the steam at a suitable point instead of allowing it to follow full stroke. All the conditions for the propulsion of vessels by steam were now in existence and experimental boats rapidly appeared. In 1783 the Marquis de Jouffroy built one which was tried at Lyons, and it is said to have been successful; but before it could be developed into a form for practical use the Revolution overtook and ruined him. At the same time John Fitch, James Rumsey, and Oliver Evans were experimenting in America. Rumsey's boats, like the proposed vessel of Dr. Allen, were fitted with jet propellers, whereby a stream of water was discharged by a steam-driven pump. His first boat was tried in Virginia in 1784 and a second, which attained a speed of 4 knots, was completed in 1786. He died in London in 1792, just previous to the trial of a new boat built from his plans. Fitch's boats were fitted with various types of propelling machinery—with paddle wheels in 1785 and afterwards with long paddles which were given motion similar to that of the paddle of an Indian canoe. In 1790 one of Fitch's boats attained a speed of 7 knots, and afterwards was used on the Delaware to carry passengers. In 1793 Filch went to France; in 1796, after returning to America, he built a small screw steamboat, but the exact measure of success that he attained is uncertain. Evans experimented with various peculiar types of steamboats, one of which was fitted with a rude screw and wheels with which to run on shore. In England Joseph Bramah obtained a patent in 1785 for propelling vessels by means of “a wheel with inclined Fans or Wings similar to the fly of a Smoke-jack or the vertical sails of a windmill.” A patent for a similar invention was issued to William Lyttleton in 1784 and to Edward Shorter in 1800. In 1791 John Stevens of Hoboken, N. J., patented a multitubular steam boiler, and he soon after began experiments with steam propulsion of boats, in which he was assisted by the celebrated engineer Mark Isambard Brunel, then an exile. Brunel left the United States in 1799, however, and it was not till three years later that Stevens completed a small screw-propelled boat which he used for his own pleasure. This little boat, only twenty-five feet in length, was the first successful screw-propelled craft built. Engines suitable for large screw steamboats were not yet invented, so that commercial success in this direction was not yet aimed at. Patrick Miller, a retired banker of Edinburgh, for several years experimented with boats of various types in a lake on his estate of Dalswinton in Dumfriesshire. These boats had two or three hulls connected by a flying deck and driven by paddle wheels placed in the space between the hulls. In the earlier experiments men were employed to turn the wheels, but in 1788, partly at the instance of James Taylor, a tutor in his family. Miller engaged a Scotch engineer by the name of Symmington to fit the boats with steam power. A small boat was tried and gave such promises of success that a larger one was built in 1789. In October of that year this boat attained a speed of seven miles an hour on the Forth and Clyde Canal. Either because of lack of interest or of means. Miller ceased thereafter to interest himself in the matter and nothing further was attempted. But in 1801 Symmington was commissioned by Lord Dundas of Kerse to build a steamer for towing barges on the Forth and Clyde Canal. This was the celebrated Charlotte Dundas. She was a success in all essential respects, but the proprietors of the canal refused to use her because they feared the effect of the wash from her paddles on the banks of the canal. She was therefore broken up and her disappointed designer turned his attention to land machinery.
The next development was Robert Fulton's Clermont, and her advent marks the beginning of steam navigation as a commercial success. In 1797 Fulton (q.v.) went to Paris from England and soon afterwards began experiments with submarine torpedoes and torpedo boats. About the year 1801 he secured the assistance of Robert Livingston, then the United States Minister to France, and they built a small steamboat. Her engines proved to be too heavy for the poorly constructed hull, which collapsed and sank. The engines were recovered, however, and placed in a larger boat 66 feet long and 8 feet broad, and on August 9, 1803, this boat was tried on the Seine, but the speed obtained was unsatisfactory. In 1804, as the agent of Livingston. Fulton went to England, where he ordered of Boulton and Watt the machinery for a much larger vessel which was to be built in the United States. In the autumn of 1806 Fulton returned to America, and the new engine followed him almost immediately. A hull, built in New York, was launched early in 1807, the engines were placed on board, and on August 7, 1807, the Clermont started on her trial trip. She proceeded without stopping to Clermont, the home of Livingston, on the Hudson, 110 miles away, and twenty hours later went on to Albany. The next day she started to New York and made the trip in thirty hours at an average speed of 5 miles an hour. Within a month she began to run regularly between Albany and New York.
The success of paddle steamers for sheltered waters was now assured, and they multiplied rapidly, particularly in the United States, where the conditions were particularly suitable. In Great Britain the use of steamers was less immediate. The first commercially successful one to be completed there was the Comet, built by Henry Bell in 1811-12. She went into service on the Clyde and was soon followed by others. In the meantime the use of steamers for ocean navigation was being tried. In 1813 Fulton began the war steamer Demologos (see United States, section on Navy), which was the first steam war vessel as well as the first ocean-going steamer. Several steamers began to make regular trips along the British coast in 1818-19, but the voyages were all short. In 1819 a vessel fitted with steam power crossed the Atlantic. This was the Savannah, of 350 tons, with a length of 100 feet, which crossed from Savannah to Liverpool in 25 days. In her, however, the engines were purely auxiliary: she was fitted with full sail power, and when the wind was fair or the seas too boisterous for steaming the paddle wheels were unrigged and taken in on deck. The beginning of real transatlantic voyages under steam was made by the Sirius and the Great Western. The latter was built for transatlantic service and was the larger and more powerful, while the former was taken from the London and Cork line. The Sirius started on April 4, 1838, and the Great Western four days later. They arrived in New York within twenty-four hours of each other, the Sirius at 10 P. M. on the evening of April 22d and the Great Western the next afternoon at three o'clock. The average speed of the Sirius was 161 miles per day—the highest 220 miles and the lowest 85 (half day only); the amount of coal consumed was 450 tons. The Great Western averaged 208 miles per day and her highest run was 247 miles. Neither vessel carried much sail.
For two or three years the transatlantic steamer service was rather irregular. The Sirius was withdrawn after making a few trips, and though the Great Western continued running, she lost money for the company that owned her. Other steamers made a few trips, but they also, like the Sirius, were withdrawn. In 1839 Samuel Cunard, of Halifax, N. S., went to England and succeeded in forming the celebrated Cunard Company. Assisted by a liberal Government mail subsidy, it was commercially successful from the start. The first vessels put in service were the paddle steamers Britannia, Acadia, Columbia, and Caledonia. They were of 1154 tons burden and their machinery was of 740 horse power. The Britannia, the first to sail, left Liverpool on July 4, 1840, and made the passage to Halifax in 12 days 10 hours; on her return voyage she did better, the time being but little over ten days. The Cunard Company enjoyed a practical monopoly of the transatlantic service until 1850, when the Collins (American) and Inman lines were started. The Collins Line lost two of its four steamers and was discontinued in 1858. Two other American steamship lines were started in 1850, the New York and Havre Steamship Company and the Vanderbilt Line. Both ceased running at the beginning of the Civil War.
The Inman Line was more successful. It began its career with steamers built of iron and propelled by screws. Though no faster than the wooden paddle-wheel Cunarders, they were cheaper to operate. The screw propeller (q.v.) now began rapidly to displace the paddle wheel, though the Cunard Company launched the Heotia, their last and finest paddle steamer, in 18G1. The rapid increase in size of ocean steamships led to the production of the Great Eastern (q.v.), but she was half a century ahead of the demands of ocean traffic and the adequate development of marine steam engineering. She was fitted with both screw and paddle engines, as it was thought impossible for either separately to deliver sufficient propulsive effect. The combination was not an economical one, and was a leading cause of her failure as a commercial venture. By 1860, in the fight for supremacy, the screw had become the unquestioned victor over the paddle wheel so far as ocean navigation was concerned, both in the merchant marine and in naval construction. Its advantages for war vessels were numerous, but the greatest of these was the possibility of placing all the propelling machinery of a screw steamer below the water line; and this alone was decisive. In merchant steamers the advantages of the screw were of a different kind. The efficiency of the paddle wheel depends upon the depth of immersion of the paddles: if too great or too little, the losses from slip, drag, and churning of the water are serious. The variation of draught consistent with economical propulsion was therefore very small—too small to admit of heavy loading. Very large wheels and feathering wheels reduced the losses somewhat, but introduced troubles of another type, while the rolling of paddle steamers in heavy seas greatly interfered with their speed and economy no matter what the character of the wheels. The draught and condition of lading of screw steamers was of much less importance and could be varied within much wider limits without perceptible loss of efficiency; rolling produced little effect, and though pitching might be serious in short vessels in which the screw was not deeply immersed, yet, owing to the small ordinary angle of pitch, the screw rarely rose high enough above the surface to give trouble.
Up to this time boilers were of the box type and the pressure of steam carried rarely exceeded 25 pounds per square inch—in many of the early steamers 10 pounds or less was the common practice. But the displacing of box boilers by cylindrical permitted a higher steam pressure, and this in turn demanded another form of engine to utilize it economically. The compound engine, which was built and patented by Hornblower in 1781 and revived by Woolf in 1804, had not been much used, because the conditions had not demanded it; but now it became a necessity. It consisted at first of two cylinders—and many compound engines are still so built—in which the steam was expanded in two stages, the first expansion taking place in the high-pressure cylinder, by which the pressure was reduced one-half, more or less, and the second expansion in the low-pressure cylinder, where the pressure was carried down to the atmospheric line or below it.
The demand for increased speed led to higher steam pressure and greater engine speed. The range of economical expansion in one cylinder being limited, the tri-compound or triple-expansion engine was designed to utilize the increased boiler pressures. The gain was two-fold. The new engines, using a higher pressure of steam, were lighter than their predecessors of equal power and they were also more economical. The first large vessel to be fitted with them was probably the Propontis, which, in 1874, was supplied with engines designed by Mr. A. C. Kirk. By 1880 the use of triple-expansion engines became common, though compound engines were largely used for another decade and they are still fitted in certain steamers where the conditions favor their economical working.
The continued demand for increased power, particularly in small vessels (torpedo boats and the like), naturally pushed up the steam pressure again, and, although the locomotive boiler was used to some extent, the advantages of the water-tube boiler soon became apparent. (See section on Boilers below.) Its capability to furnish very high pressure steam reacted upon engine design and produced the quadruple-expansion engine. The water-tube boiler is not yet much used in the mercantile marine, but is rapidly displacing the cylindrical boiler in naval construction. It has not yet brought about the extended use of quadruple-expansion engines in large vessels, but this may follow in the course of time.
The length of the voyage and the vast amount of traffic has caused the transatlantic trade to be the principal field of steamship development. While the gain in size and speed of the vessels in this trade has been continuous from the start, a great impulse was given by the building of the Britannic and Germanic for the White Star Line in 1874. They at once reduced the average passage from Queenstown to New York to about eight days. They were followed in 1870 by the Arizona of the Guion Line, confessedly built to outstrip all competitors, and her success was the beginning of a race for speed supremacy that has brought out a new ‘record-breaker’ almost every year since. As the vessels are increased in size it takes less horse power per ton to drive them at a given speed, and this has tended to augment the tonnage of the great liners so that the dimensions of the Great Eastern are now exceeded by nearly a dozen vessels, built and building. The accompanying table gives the principal features of the largest ships belonging to the-great steamship companies, transatlantic and transpacific.
Table Giving Particulars of the Largest and Most Important Steamers in the World
|NAMES OF VESSELS AND OF OWNERS||When
|(Built 1888, rebuilt 1901)|
|Atlantic Transport Line:|
|Compagnie Générale Transatlantique:|
|North German Lloyd Line:|
|Kaiser Wilhelm der Grosse||1898||5,521||14,349||20,880||625.0||648.6||66.0||43.0||28.0||30,000||23.0|
|Kaiserin Maria Theresa||1899||3,769||7,800||.........||.......||546.0||52.0||37.0||......||17,000||21.0|
|Kaiser Wilhelm II||1902||.........||19,500||26,000||.......||706.5||72.0||52.5||29.0||39,000||23.5|
|Red Star Line:|
|White Star Line:|
|Great Northern Railway Company (Transpacific):|
|Pacific Mail Company (Transpacific):|
N.B. — The names of vessels which fly the American flag are printed in italics.
A TRANSATLANTIC LINER—NORTH GERMAN LLOYD STEAMSHIP “KAISER WILHELM II.”
ONE OF THE TWO SETS OF MAIN ENGINES OF THE “KAISER WILHELM II.”
Each set of engines develops 20,000 horse-power, and consists of two four-cylinder, three-crank, quadruple-expansion engines.
Marine Engines. Marine engines have during recent years tended to one general type. For special services on inland waters a great number of various specialties are found, but in sea-going steamers the type in almost universal use is the vertical (i.e., the piston moves vertically), inverted (i.e. the cylinder is above the crank), direct-acting (i.e. the connecting rod joins the crosshead directly to the crank), triple (or quadruple) expansion engine. Engines differ as regards fittings and attachments, length of stroke and revolutions, weight, speed, etc., but a description of the general type will give the essential features of all.
Naval engines are built lighter, have a shorter stroke, and run at higher speeds than those in the merchant service. In a triple-expansion engine the steam works expansively in three successive stages, in a quadruple in four. The reason for the introduction of the multiple-expansion engine is the greater economy obtained when steam is used expansively over a greater range. This cannot be efficiently accomplished in a single cylinder owing to various causes chiefly due to liquefaction, hence compound (two stage) engines were introduced, then the triple and quadruple. The economy gained by the compound over the simple is about 50 per cent., by triple over compound about 25 per cent., and by quadruple over triple about 10 per cent. In the quadruple the gain in economy is obtained by considerable increase in weight, so that for most services the real gain of the quadruple is questionable. The type of engines will depend largely on the steam pressure employed. For a pressure of 40 to 90 pounds a compound; up to 190 pounds the triple; above 190, the quadruple may be used if space and weight are not very important. At present the pressure used is ordinarily between 140 and 250 pounds per square inch above the atmosphere. A great advantage of multiple-expansion engines, and a cause for their adoption, is the more even turning effect and better balancing obtained. The triple-expansion engine has either three or four cylinders, more often three, arranged in successive order, H.P. (high pressure), I. P. (intermediate pressure), L.P. (low pressure), each cylinder being attached by means of its piston and connecting rod to its own crank on the crank shaft, which is usually made in interchangeable sections, one for each crank. Cranks are usually set at 120° from each other to obtain even turning effect. Four cylinders are used when the L. P. cylinder would be too large to be conveniently fitted or built, or to obtain a better balancing of the engine and reduce vibrations. The sequence of the cylinders is then high pressure, intermediate pressure, low pressure, low pressure; or, on what is known as the Yarrow-Schlick-Tweedy system, low pressure, high pressure, intermediate pressure, low pressure, with cranks set at right angles. Here the crank shaft is generally in two sections.
The course of steam in a triple-expansion engine would be as follows: Leaves main steam pipe, passes through separator, then throttle, and into high-pressure valve chest. The movement of the valve opens and closes the steam ports at pressure cylinder for 0.6 to 0.75 of the stroke and then cuts off. The steam in the cylinder then expands, continuing to move the piston. Just before the end of the stroke the valve opens to exhaust and at about the same time begins to allow steam to enter on the other side of the piston; this results in cushioning at the end of the stroke. The steam having been reduced about 50 to 60 per cent. in pressure and correspondingly in temperature, leaves the high pressure cylinder, and passes to the intermediate pressure receiver, which is really the intermediate pressure valve chest. From here it enters through the intermediate pressure valve and does its work in the intermediate pressure cylinder, being again reduced in temperature and pressure. On leaving the intermediate pressure cylinder the steam is generally at about atmospheric pressure. Then it is conducted to the low pressure receiver and goes through its third stage of working and expansion. On leaving the low pressure it goes through the exhaust pipe to the main condenser where it is condensed, and then as water and vapor it is pumped by the air pump to the hot well or feed tank, and thence to the boiler, where it is reëvaporated.
|SECTIONS OF ENGINE OF UNITED STATES TWIN-SCREW PROTECTED CRUISER.|
|Displacement of Vessel, 3200 tons.||Diameter of cylinders, 18, 29, and (2) 35 1-2 Inches.|
|Indicated Horse-power, 4500.||Stroke, 30 inches.|
A plate illustrating a four-cylinder triple-expansion engine of a modern second-class cruiser for the United States Navy is shown with various parts indicated. The engine framing is supported on a bed plate made in sections and bolted by holding-down bolts to structural parts of the vessel. The engine framing is made of steel columns braced by various cross rods. A more general practice is to have cast or wrought steel inverted Y frames on one side which support the guides, and in the merchant service the condenser is generally cast in one piece with lower portions of the Y frames. The cylinders are supported on top of framing and bolted to it by various fastenings. In large engines each cylinder is a separate casting; generally the valve chest is cast with the cylinder, making the whole a rather intricate casting. Cast iron is generally used for cylinders. Cylinders are fitted with liners, a, which form the bearing surface for the piston. The liners are bolted at the lower end to the bottom of the cylinder and the joint at the top is packed. Liners are of cast steel or cast iron; steel is stronger, but cast iron gives a better wearing surface. The space between the cylinder and liner is commonly used as a steam jacket. Lately the economy of steam jacketing, especially for fast-moving engines, has been questioned. All cylinders are fitted with covers, c, of cast iron or steel, and these are secured to the cylinders by bolts and nuts and the joints packed by gaskets. For large cylinders, smaller openings called bull's-eyes, d, are fitted for purposes of examination. The pistons are cone-shaped, made of forged or cast steel or cast iron, and fitted with cast iron spring packing rings, c. The piston rod is secured to the piston by its taper and the piston rod nut, f, on top. The opening in the bottom of the cylinder for the piston rod is fitted with a stuffing box, g, supplied with some form of metallic packing. The lower end of the piston is secured to the crosshead h, made of forged steel, which has a slipper, i, sliding on the crosshead guide, j. The crosshead ahso has journals, k, for the upper end of the connecting rod. The lower end of the connecting rod is attached to the crank pin, l, by means of the crank pin brasses, m. The crank shaft is supported by the main bearings, n, which are supported by a bed-plate. All large bearings are lined with anti-friction metal.
The valves o for the H. P. and I. P. cylinders are piston valves, either single or double ported, hollow or solid. For the L. P., the double-ported flat slide valve, p, fitted with a relief ring is used. Piston valves are employed with high pressure because in them the pressure is on all sides and there is no force holding the valve against its seat. They are fitted with spring rings to make them tight. Valves are made of cast iron, cast steel, or forged steel. The valve seats are generally liners of cast iron. The valve stem is secured to the valve by its taper or shoulder and the nut q and the upper end r of the rod is fitted as a guide. The lower end of the valve rod is connected by means of a bearing to the link block s, which works in the link t. To the ends of the link are attached the eccentric rod u, and the lower ends of these rods are bolted to the eccentric straps v, which move around eccentrics w. The eccentrics are secured to the shaft and fitted so that the position can be slightly changed. There are two eccentrics, one to give go-ahead motion and the other backing. The link is moved by an arm attached to the reversing shaft x, which is operated by the reversing engine y, and this engine is controlled by the reversing lever. The link arm is attached to the adjustable cut-off block, by means of which the cut-off can be varied from .5 to .75 of stroke. This is the Stephenson gear, which is most generally used; others are Marshall's, Joy's, Morton's, etc.
In the cut shown, the air pump A is operated by a cross beam B attached to the L. P. crosshead. In many merchant vessels the circulating, bilge, and feed pumps are operated from such a beam. For large installments these pumps are as a rule independent and of the Blake, Worthington, Snow, or other patent type. The office of the air pump is to pump the condensed water and vapor from the condenser to the feed tank and produce a vacuum. Surface condensers are now always fitted, and the steam and condensed fresh water are kept separate from the circulating sea water. This keeps salt out of the system. The condenser consists of an approximately cylindrical vessel, having a water chamber and a tube sheet at each end. Brass tubes connect the two tube sheets and cold sea water is pumped through the tubes by means of the circulating pump, thus cooling and condensing the exhaust steam surrounding the tubes. Condensers are made of bronze or cast steel and the tubes of brass. Circulating pumps are centrifugal and operated by a vertical simple or compound engine.
The feed tank is generally fitted with a filter chamber for purifying the water. Feed heaters, using auxiliary exhaust steam to heat the water before reaching the boilers, are fitted for purposes of economy and make the service less hard on boilers. The feed pumps are vertical, single, or duplex plunger pumps. Cylinders are lagged (i.e. covered with non-heat-conducting material) to prevent loss of heat. All cylinders are fitted with relief valves set at appropriate pressures, drains for conducting off any water that may accumulate, and indicator pipes, cocks, and reducing motion for taking indicator cards. Pressure gauges are supplied to indicate the pressure in the steam pipe, the various receivers, and the vacuum in the condenser. Revolution counters are attached, which automatically record the number of revolutions of the engines. A water service is supplied, consisting of a system of piping by means of which sea water can be circulated through such parts as the thrust bearing and crosshead guides, or sprayed on various other bearings where heating is likely.
The oil services on a modern engine are very elaborate, as all working bearings must be supplied with a lubricant. The best practice is the manifold system, where each bearing has a small pipe leading up to one of the several manifolds where it is fed by means of a wick. The manifold can be filled from a reservoir placed above the level of the engine. Besides oil, graphite and various preparations of tallow and grease are used for lubrication.
The steam pressures now used are 150-300 pounds. It is not likely that much higher pressures than 250 will be soon used, on account of the great strength of parts necessary to withstand the pressure, the difficulty of keeping tight joints, and the high temperature of steam, which heats the working surfaces and prevents proper lubrication.
Steam is expanded in triple-expansion engines 6 to 9 times; in quadruple, 8 to 12 times. The ratio of the area of the H. P. to that of L. P. cylinder varies from 1 to 5 to 1 to 10, there being a greater ratio with increased pressures. The revolutions vary from 80 or 100 per minute in very long stroke engines to 400 to 500 in high-speed torpedo boats. The piston speed is limited to about 1000 feet per minute. The length of stroke for large merchant vessels is four to six feet; for naval engines not over four feet; with smaller engines the stroke is less. A relatively long stroke results in greater economy.
Of late the steam turbine is beginning to be introduced in place of the reciprocating engine for fast vessels. See Steam Turbine.
Boilers. Modern marine practice is either to use the cylindrical fire-tube boiler carrying pressures of 150 to 200 pounds per square inch, or some form of water-tube boiler using pressures of 160 to 300 pounds. W.T. (water-tube) boilers are more largely used for naval purposes and fast passenger vessels and cargo vessels in fresh water, cylindrical boilers for general merchant service.
The substantial advantages of the cylindrical boiler are: reliability, simplicity; it is well made and generally understood; it can use salty or dirty water, and it will stand hard usage without serious loss or injury. The disadvantages are: great weight; steam cannot be gotten up or taken off quickly; it does not readily adjust itself to change of output; and heavy forced draught cannot be used.
The advantages of the water-tube boiler are: lightness; adaptability to high pressure; rapidity of raising steam or taking it off; it is readily adjusted to sudden change of output; forced draught can be used (in nearly all types); and repairs or removals are more easily made. Its disadvantages: it requires great care and attention; it cannot use salty or dirty water or experience hard usage; corrosion takes place very readily; it is complicated and many types require a large number of mechanical attachments; and being new, it is not well understood by men who handle it and best results are not obtained. The economy of fuel is about the same in the best of each type. For average running the cylindrical is probably the more economical.
The general form of a cylindrical boiler is the single or double ended return-tube boiler fitted with two, three, or four corrugated furnaces. Boilers vary from 9 to 20 feet in diameter, and 9 to 18 feet in length for single-ended, and 17 to 21 feet for double-ended. A cut of a single-ended boiler is shown; aa are the shell-plates, made in two courses of three sections each, with butt joints; bb, corrugated furnaces, either of Fox, Purves, or Morrison patent, into which the grate and bridge wall are fitted; c, stays supporting boiler front and combustion chamber; DD, tube sheets with tubes, e, expanded into them; some of tliese are stay tubes screwed into tube sheets which they help to support. F is the combustion chamber, where the gases of combustion are finally mixed and burned. The products of combustion pass through the tubes to the uptake and then to the smoke pipe. The floating surface is composed of the crown sheets (top of furnaces), top and sides of combustion chamber, and tube surface, the tube surface being by far the larger portion. H, H, H are steam space stays supporting the boiler ends. The back and sides of the combustion chamber are supported by short stay bolts, I, and the top by girder stays, J. The steam space is fitted with a dry pipe which collects the steam and discharges through the stop valve to the steam pipe.
SINGLE-ENDED CYLINDRICAL BOILER.
The furnace front is fitted with furnace and ash-pit doors, the ash pit being the part of the furnace below the grate. Manholes, K, are fitted to obtain access to boiler for cleaning, etc. The coverings for manholes are called manhole plates. Practically all parts of a boiler, except the grate, furnace doors, and bridge wall, are built of mild steel.
The attachments of a boiler like the one shown are: main and auxiliary stop valves in the steam pipe; check valves through which feed water enters; surface and bottom blow valves, by means of which the boiler is blown down or pumped out; two water columns to show the height of the water; pressure gauge; spring safety valve; and circulating apparatus (generally a hydrokineter).
The grate surface of such a boiler develops 13 to 16 indicated horse power per square foot. Ratio of heating to grate surface, 30-35 to 1. Weight per I. H. P. including water, 90 to 120 pounds. Maximum coal burnt per square foot grate surface per hour about 40 pounds ordinarily 15 to 20 pounds. The efficiency of the boiler is about 70 to 75 per cent. in best condition.
BELLVILLE BOILER—ECONOMIZER TYPE.
In water-tube boilers the water is contained within the tubes, and as these are relatively small great pressures can be carried and the boiler may be considerably lighter. Most types of water-tube boilers have a number of steam and water chambers connected by a system of tubes either straight or bent. The feed water usually enters the upper or steam drum and is conducted by down tubes to a lower or water drum; from here the water, becoming heated, rises and passes up through steam-collecting tubes to the steam drum. Thus a circulation is set up. The efficiency of a water-tube boiler depends in a large measure on proper circulation. As the distance of grate to smoke stack is rather short, most types of water-tube boilers have a system of baffle plates for conducting the gases among the tubes to increase the distance of travel. The economy depends in large measure on efficient baffling.
The boilers are fitted with a casing made of fire brick, asbestos, or other non-conducting material held in place by thin sheet metal.
Water-tube boilers have all the attachments enumerated for the cylindrical fire-tube boiler. In addition nearly all except Babcock & Wilcox boilers have automatic feeding apparatus. Bellville boilers are fitted with reducing valves. Some types, especially Bellville, are fitted with feed heaters or economizers placed above the boiler proper, where the feed water is heated before entering the boiler. Others are fitted with superheaters. Most types have an arrangement of steam or air service for the removal of soot.
Down-tube boilers are those in which the steam-generating tubes discharge into a steam drum below the water line. Priming boilers are those where these tubes discharge at or above the water line. Such a boiler as the Schultz appears to be neither one nor the other.
Large-tube boilers use tubes varying from three to five inches in diameter. Small-tube boilers use tubes one to two inches in diameter.
Express boiler is a term applied to rapid steaming small-tube boilers, capable of large power on small weight and using heavy forced draught. This type is rather less economical and is chiefly used for very fast vessels such as torpedo boats.
Up to the present time the Babcock & Wilcox and Dürr of large-tube type, and the Thornycroft, Yarrow, and Normand of express type, have given the greatest satisfaction in service. The Niclausse and Bellville appear to give satisfaction in some services and dissatisfaction in others.
THORNYCROFT BOILER—DARING TYPE.
The Babcock & Wilcox and Bellville are straight-tube priming boilers. The former is shown in section on the plate accompanying the article on Boiler (q.v.). The Niclausse, Dürr, Yarrow, and D'Allest are straight-tube ‘drowned’ tube boilers. The Thornycroft and the Schultz are express bent-tube priming boilers. The Normand is an express bent-tube ‘drowned’ tube boiler. Of these the Yarrow, D'Allest, and Babcock & Wilcox are simplest and the Bellville most complicated.
The systems of forced draught in use are the closed ash pit and closed fire room. Of induced systems generally fitted with air heaters we have various patent forms. Superheating is somewhat used, but does not meet with great satisfaction, owing to increased cost and weight and the rapid deterioration of such attachments.
The economic results and other data of water-tube boilers varies so much with different types and conditions that no average results can fairly be taken. As a rule there is a larger ratio of heating surface than in Scotch and a decrease in weight, so that among some of the best types the weight per indicated horse power, including water, is 50 to 90 pounds. See Shipbuilding; Steam; Steam Engine; Ship, Armored; Navies; Transportation.